Depends on whether one reads "between 1 & 4" as [1,4] (inclusive; which is what the regex notation used implements) or (1,4) (exclusive; which would be {2,3}). Given that the inclusive sense was implied by the code itself one could probably overlook it (of course there's a huge potential for debate on whether an unqualified between should be read as one or the other . . . :)

(And if you want pedantic nits to pick, one could also have knocked everyone's use of [0-9a-f] rather than [[:xdigit:]] . . . :)